This invention relates to ion trap mass spectrometers and more particularly to controlling and reducing space charge effects in such mass spectrometers.
Conventional ion trap mass spectrometers, of the kind described in U.S. Pat. No. 2,939,952, are generally composed of three electrodes, namely a ring electrode, and a pair of end cap electrodes. Appropriate applied RF and DC voltages are applied to the electrodes to establish a three dimensional field which traps ions within a specified mass-to-charge range. Linear quadruples can also be configured as ion trap mass spectrometers where radial confinement is provided by an applied RF voltage and axial confinement by DC barriers at the ends of the rod array. Mass selective detection of ions trapped within a linear ion trap can be accomplished by ejecting the ions radially, as taught by U.S. Pat. No. 5,420,425, or by ejecting the ions axially, as taught by U.S. Pat. No. 6,177,668. Ions may also be detected in situ using Fourier Transform techniques, as taught by U.S. Pat. No. 4,755,670.
The performance of any ion trap mass spectrometer is strongly influenced by the trapped ion density. Whenever this ion density increases above a particular limit, the resolution and mass assignment accuracy degrade. In extreme cases the mass spectral peaks can be completely smeared out and little useful information obtained. Accordingly, it is desirable to provide a method for rapid determination of the ion current provided by the ion source so that the number of ions injected into a linear ion trap mass spectrometer can be adjusted for optimal mass spectrometry performance.
Linear ion trap mass spectrometers are variations of 2-dimensional quadruple mass spectrometers or other multiple devices, which allow ion trapping by means of a two-dimensional quadruple, or multiple, field applied in the radial dimension and DC barriers applied at the ends of the device. Such linear ion traps may be fabricated from straight or curved rod-type electrodes. Quadruple ion traps, at least, then permit mass selective ejection from the quadruple followed by ion detection. U.S. Pat. No. 6,177,668 teaches that the ion path of a standard triple quadruple mass spectrometer can be configured such that one of the quadruples can be operated as a linear ion trap mass spectrometer. Such an instrument offers the capabilities of both an ion trap operational mode with the associated high sensitivity and the conventional operation mode of a standard triple quadruple mass spectrometer on the same platform, which is an advantage. The present inventor found that by combining the capabilities of both standard triple quadruple and linear ion trap modes a very rapid method of space charge minimization can be obtained. The invention is, in general, applicable to any linear ion trap capable of operating in both a trapping mode and a continuous transmission mode.
In accordance with the present invention, there is provided a method of setting a fill time for a mass spectrometer including a linear ion trap the method comprising:
(a) operating the mass spectrometer in a transmission mode;
(b) supplying ions to the mass spectrometer;
(c) detecting ions passing through at least part of the mass spectrometer in a preset time period to determine the ion current;
(d) from a desired maximum charge density for the ion trap and the ion current determining a fill time for the ion trap;
(e) operating the mass spectrometer in a trapping mode to trap ions in the ion trap, and filling the ion trap for the fill time determined in step (d); and
(f) obtaining an analytical spectrum from ions trapped in the ion trap.